1. Field of Invention
The invention pertains to wavelength-agile laser transmitters. More particularly, the invention pertains to tunable lasers that may include internal wavelength referencing that can be adjusted to selectable charnels on a wavelength grid, and to re-configurable optical communications networks.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Optical fiber has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Optical fiber is capable of carrying much more information than copper with lower attenuation.
Currently expansion of bandwidth in fiber optic networks is being accomplished by what is known as “wavelength division multiplexing” (WDM), in which modulation techniques are used to handle separate subscriber/data sessions concurrently on a single optical fiber. Current International Telecommunications Union (ITU) specifications call for channel separations of approximately 50 GHz. The ITU grid typically refers to those frequencies centered about approximately 1550 nm (e.g. approximately 1525 μm to 1575 nm for the C band). At those wavelengths, 50 GHz channel spacing corresponds to a wavelength separation of approximately 0.4 nm. In WDM applications, each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is then combined onto a single optical fiber for transmission.
One type of laser presently used for fiber optic communications is a distributed-feedback (DFB) diode laser. In a system employing such a DFB laser, the wavelength grid may be defined by the transmission peaks of a reference etalon. Such a system would typically use a combination of the DFB laser and the reference etalon in a feedback control loop. DFB lasers suffer from the drawback that they have small tuning ranges of ˜3 nm (See, e.g., J. Carroll, J. Whiteaway, & D. Plumb, Distributed feedback semiconductor lasers, SPIE Press, 1998). DFB laser transmitters must be individually sorted and each laser is usable for only one channel or a small number of adjacent channels.
On the other hand, WDM networks are evolving towards re-configurable architectures in which each transmitter's wavelength must be re-selectable on command. Re-configurable networks offer significant capacity, reliability, and management advantages over static systems (See, e.g., R. Ramaswami and K. Sivarajan, Optical Networks. A Practical Perspective, Morgan Kaufmann Publishers, 1998).